Device and method for dilution of cellulose pulp

ABSTRACT

A device and a method for dilution of cellulose pulp of a consistency of between 20% and 45%. The cellulose pulp is arranged to fall down in a pulp chute during an addition of dilution liquid. The dilution is performed to a consistency in the range of 6-18%, above a level of established diluted pulp in a bottom of a pulp chute. A central distribution device with nozzles for addition of dilution liquid to the falling pulp above the level of established diluted pulp in the bottom of the pulp chute is arranged in the center of the pulp chute. The nozzles of the distribution device are fed with dilution liquid via a pipe that is arranged at least partly through the pulp chute.

The present invention relates to an improved device for dilution ofcellulose pulp in the form of flocs and to a related method. Thedilution is intended to be performed in order to dilute HC-cellulosepulp of an original consistency of about 20 to 45% (HC=HighConsistency), the dilution being withheld until the formation of aMC-pulp, i.e. until the pulp reaches a consistency in the range of 6-18%(MC=Medium consistency).

BACKGROUND

In connection to the dilution of decomposed cellulose pulp of a highdryness, typically a consistency of above 20%, it is normal procedure toadd dilution liquid whilst agitating by means of a dilution screw. Thishas also been the starting point in the Swedish patent SE 526292, whichrepresents the prior art. Within this prior art dilution to the desiredconsistency is achieved by the addition of dilution liquid via a ringshaped supply chamber around a pulp chute.

A ring shaped supply chamber for the addition of dilution liquid hasbeen used in the business for a long time and has for instance beenutilized for the dilution of cellulose pulp after HC-Ozon reactors, i.e.bleaching reactors in which cellulose pulp of high consistency, i.e.above approximately 20%, is bleached with Ozon. A HC-Ozon reactor isconventionally a horizontal reactor with an interior transport andmixing screw, which normally is arranged directly after a shreddingscrew from a wash press, and in which the pulp falls down into a pulpchute after having been treated in the HC-Ozon reactor. In e.g. U.S.Pat. No. 5,174,861, the fundamentals of a horizontal. Ozon reactor isdisclosed, in which the HC-pulp falls freely down into a dilution tankafter the reactor.

It has been noticed that the use of a ring shaped supply chamber arounda pulp chute that periferally injects dilution liquid towards thefalling floes of pulp, only achieves a certain degree of penetrationinto the flow and that the maximum quantity of dilution liquid that maybe added is limited.

In a plant with a capacity of 3000 ADT (Absolutely Dry Tons) of pulp aday, a pulp chute of a diameter in the range of 1.5 metre is utilizedand conventionally no pulp chute for MC-pulp (Medium Consistency) with adiameter of less than 0.80 metres has been utilized. This means that avery good agitation of the pulp is required in order to achieve asatisfactory dilution by means of a ring shaped supply chamber.

SHORT DESCRIPTION OF THE INVENTION

The present invention relates to an improvement of the dilution withrespect to a ring shaped supply chamber. An object of the invention isto overcome the problems of the prior art and to evenly distribute thedilution such that substantially the whole pulp volume will have thesame consistency. This is achieved by means of a device according toclaim 1.

According to a first aspect, the invention relates to a device fordilution of cellulose pulp of a consistency of between 20 and 45%, whichdevice comprises a pulp chute through which cellulose pulp is arrangedto fall down under the addition of dilution liquid, wherein the dilutionis performed to a consistency in the range of 6-18%, and wherein theaddition of dilution liquid is performed on the falling pulp above alevel of established diluted pulp in the bottom of the pulp chute.Further, a central distribution device with a nozzle arranged in thecentre of the pulp chute for the addition of dilution liquid to thefalling pulp in the centre of the pulp chute and above the level ofestablished diluted pulp in the bottom of the pulp chute, wherein thenozzle is fed with dilution liquid via a pipe that is arranged at leastpartly through the pulp chute.

According to a second aspect, the invention relates to a method ofdilution of cellulose pulp with a consistency of between 20 and 45%,where dilution liquid is added to the cellulose pulp in a pulp chute,which dilution is performed to a consistency in the range of 6-18% asthe cellulose pulp is in free fall above an established level of dilutedcellulose pulp at the bottom of the pulp chute. The dilution liquid isadded in the centre of the pulp chute.

In the device and the method according to the invention flocs of pulpare wetted throughout essentially the whole cross section of the pulpchute before the flocs hit the established pulp level in the chute.Thus, not only the flocs of pulp falling along the walls of the pulpchute will be wet, in which case the degree of wetting decreases withincreasing distance from the walls of the pulp chute and is close tozero in the centre of the pulp chute. Instead, an even dilution, whichis especially important with increasing capacity of the plants, and withthe invention it is possible to reach a capacity of 4-5000 ADT pulp perday and plant.

With the invention it is possible to obtain a higher degree of homogendilution of cellulose pulp in free fall, since more dilution liquid maybe added, which furthermore may be more evenly distributed in the pulp.Also, it is possible to obtain a high degree of homogenously dilutedcellulose pulp already in the free fall, i.e. without agitating theliquid. Another advantage is that it is possible to divide the dilutionliquids such that a less aggressive dilution liquid may be added in theperiphery as a complement to treat the walls of the pulp chuteleniently, wherein a less costly material may be used for these, whilemore aggressive pH adjusting chemicals, such as acids or bleachingchemicals, are added in the centre of the chute and surrounded by aprotecting curtain of less aggressive cellulose pulp, closer to theinner walls of the chute.

SHORT DESCRIPTION OF THE DRAWINGS

Below, an embodiment of the invention will be described in detail withreference to the accompanying figures, of which:

FIG. 1 schematically shows a comprehensive view of an embodiment of adilution device according to the invention;

FIG. 2 shows a view from below of an embodiment of the invention;

FIG. 3 shows a cross sectional side view of an embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a position in the pulp line where decomposed cellulose pulpin the form of flocs is transported in a screw feeder 1 in which a feedscrew 2 feeds flocs of cellulose pulp to a pulp chute 3. The screwfeeder 1 may constitute the final part of a pulp shredder or transportscrew in a bleaching reactor in which the cellulose pulp is bleached ata high consistency, above at least 20%.

In the upper part of the pulp chute 3 a dilution device 10 is arranged,which adds dilution liquid to the flow of flocs that falls freely downtowards an established level of pulp 6 in the pulp chute 3. The dilutiondevice 10 comprises a line 15 that is drawn to a central distributiondevice with nozzles 11 arranged in the centre of the pulp chute 3 inorder to distribute dilution liquid to the cellulose pulp, as it fallsfreely in the centre of the pulp chute. Additionally, in the shownembodiment there is also a supply chamber 16, which is arranged foradding dilution liquid concentrically around the pulp chute, and whichis provided with a plurality of nozzles 14 on the inner wall 3A of thechute 3 (see FIG. 2) for distributing dilution liquid to the flocs thatfalls freely.

The diluted pulp, which by the addition of dilution liquid has obtaineda relatively homogenous concentration, forms an established level 6 ofdiluted pulp in the bottom of the chute 3. In connection to the bottomof the chute a MC-pump 4 is arranged, which in a conventional manner isprovided with a fluidizer 5 in front of the pump wheel of the pump. Thefluidizer 5 is at least partly arranged inside the chute 3 and providesa hefty agitation of the cellulose pulp in the bottom of the pulp chute,which is indicated with rotational arrows, and accomplishes thehomogenisation of the concentration of the cellulose pulp. Normally, thefluidizer is integrated with the pump wheel of the pump, but it may alsobe separated from the pump wheel and be provided with an own drivingmeans, and placed in immediate connection of the inlet to the pumpwheel.

In FIG. 2, an embodiment of the dilution device 10 is shown in a viewfrom below. Dilution liquid is added via the line 12, wherein thedilution liquid reaches a certain pressure inside a supply chamber 16defined by an outer wall 10A and the inner wall 3A the pulp chute 3. Anumber of outlets 14 are arranged in the periphery of the pulp chutefrom the supply chamber 16 and directed inwards, towards the centre ofthe pulp chute. The pressure in the supply chamber 16 is such that thedilution liquid is sprayed into the pulp chute towards the falling pulp.The pressure control, the shape of the outlet and the quantity ofoutlets may vary in a number of different ways, which are evident to theskilled person. The object is however to yield maximal penetration intothe flow of falling pulp flocs.

As is apparent from the figures, the central distribution devicecomprises at least one pipe 15 extending from the wall 3A of the pulpchute 3 inwards, towards the centre of the pulp chute. At the end ofthis pipe 15, at least one nozzle 11 is arranged, for the addition of atleast a part of the dilution liquid. The nozzles 11 may have severaloutlets and may distribute the dilution liquid in form of a downwardlydirected shower stream 11A with a substantially conical jet (see FIG.3). However, the dilution liquid may also be added in a directionobliquely upwards or completely horizontally outwards towards the walls3A in the pulp chute. An advantage of adding the dilution liquidobliquely upwards is that it thus is given more time for mixing with thefalling pulp, whereby it of course mixes better. Another advantage isthat the energy of impact from the dilution liquid hitting the pulpcontributes in mixing the pulp, which leads to that the pulp mixing getsa more homogenous consistency. In order to further improve thedistribution of dilution liquid to the falling cellulose pulp it isfeasible to arrange several central nozzles 11, which are distributedover substantially the whole width of the pulp chute.

An alternative or complement to such a distribution of the nozzles 11 inthe centre of the pulp chute 3 is to, as in the shown embodiment, alsoarrange peripheral outlets 14. In FIGS. 1 and 3, the nozzle 11 of thecentral distribution device is arranged below the peripheral outlets 14of the supply chamber 16 in the wall 3A of the pulp chute. In this waythe addition of dilution liquid is directed towards the upper side ofthe pipe 11 of the central distribution device, partly in order to keepthis free from flocs that otherwise easily may gather thereon.Therefore, as is shown in FIG. 3, one of the jets 14B from theperipheral outlets 14 may directed in parallel with the upper side ofthe pipe 11. The pipe may also be drawn into the pulp chute in the waythat is shown in FIG. 3, i.e. obliquely downwards such that the flocsfalling from above towards the pipe shall fall of more easily. To askilled person it is obvious that the pipe 15 also may be drawn inseveral other fashions. It is of course important that the pipe does notbinder the falling the pulp too much, a certain contact may however bepositive since it may function as an agitation of the pulp, which maycontribute to better mixing and homogenisation of it.

In one embodiment of the invention the feeding pipe 15 is provided witha separate feeding, wherein the outlets or nozzles 11 of the centraldistribution devices may be fed with a different dilution liquid fromanother source than the one that is being fed to the peripheral nozzles14. Thus, it is possible to add a pH-regulating agent, bleachingchemicals or other additives centrally in the pulp chute, whereby thefalling flocs of pulp that has been totally or partly diluted with aless aggressive dilution liquid shields off the centrally added dilutionliquid such that it does not reach the inner wall 3A of the pulp chute.The design of the supply chamber 16 and the upper parts of the pulpchute 3 may hence be designed in a simpler and cheaper material, whereonly the pipe 15 and nozzle 11 of the central distribution devicerequires materials that are resistant to a concentrated dilution liquid.

Large quantities of dilution liquid are added to the flow of freelyfalling pulp flocs and may be followed by air or gas in the flow,wherein an active deaeration may be needed in order to vent redundantvolumes accompanying the flow. The pulp chute may therefore be designedwith a deaeration canal 13 that is connected to the pulp chute above thesupply chamber 10. The deaerated gases may either be released to thesurrounding atmosphere or to a decomposition system depending on whetherthe gases are environmental harmful or not.

1. A device for dilution of cellulose pulp of a consistency of between20% and 45%, comprising: a pulp chute through which cellulose pulp isarranged to fall down, means for adding the dilution liquid on thefalling pulp above a level of established diluted pulp at a bottom ofthe pulp chute to dilute the consistency to a range of 6-18%, a centraldistribution device with a nozzle arranged in a center of the pulp chuteadapted to add dilution liquid to the falling pulp in the center of thepulp chute and above the level of established diluted pulp at the bottomof the pulp chute, and means for feeding the nozzle with dilution liquidvia a pipe that is arranged at least partly through the pulp chute. 2.The device according to claim 1, wherein the central distribution devicehas several nozzles that are evenly distributed over a width of the pulpchute.
 3. Device The device according to claim 1 wherein a peripheraldistribution device with peripheral nozzles is arranged for addingdilution liquid from an inner wall of the pulp chute.
 4. The deviceaccording to claim 3, wherein the central distribution device isarranged below the peripheral nozzles of the peripheral distributiondevice.
 5. The device according to claim 4 wherein the device furthercomprises means for separating the feeding to the nozzles of the centraldistribution device from the feeding to the peripheral nozzles of theperipheral distribution device, and means for feeding dilution liquidfrom different sources to the peripheral nozzles of the peripheraldevice and the nozzle of the central distribution device.
 6. The deviceaccording to claim 5 wherein a deaeration canal is arranged inconnection to the pulp chute above the nozzle of the centraldistribution device and the peripheral nozzles of the peripheral device.7. A method for dilution of cellulose pulp having a consistency ofbetween 20% and 45%, comprising: adding a dilution liquid to thecellulose pulp in a pulp chute, which dilution is performed to aconsistency in a range of 6-18% as the cellulose pulp is in free fallabove an established level of diluted cellulose pulp at a bottom of thepulp chute, and adding dilution liquid in a center of the pulp chute. 8.The method of claim 7 wherein the dilution liquid is added from severalnozzles that are substantially evenly distributed over a width of thepulp chute.
 9. The method of claim 7 wherein dilution liquid also isadded from an inner wall of the pulp chute.
 10. The method of claim 9wherein different dilution liquids are added in the center and aperiphery of the pulp chute, wherein the dilution liquid added in thecenter of the pulp chute is more concentrated than the dilution liquidadded in the periphery of the pulp chute.
 11. The method of claim 7wherein at least 25% of the dilution liquid is added in the center ofthe pulp chute.